The present invention relates to a method of producing a paperboard or cardboard product that possesses a strengthened glue joint. In particular, this invention relates to a method, and the paperboard or cardboard products formed thereof, of adhesively bonding opposed paperboard surfaces together by separately applying a styrene-butadiene, or a modified styrene-butadiene, latex adhesive and an adhesive that possesses good green strength, such as a vinyl acetate latex adhesive, to the bonding portion of a paperboard surface. This invention further relates to adhesively bonding opposed paperboard surfaces together by applying a styrene-butadiene, or a modified styrene-butadiene, latex adhesive to the bonding portion of a paperboard surface and to paperboard and cardboard products formed thereof.
Adhesives, such as water glass, hot melt glue, protein glues, vegetable adhesives and synthetic adhesives, are widely used in the fabrication of paperboard and/or cardboard products, such as containers. For instance, starch-based adhesives having various additives such as urea-aldehyde, ketone-aldehyde, urea-formaldehyde or melamine-formaldehyde resins or polyvinyl acetate latex to render the starch-based adhesives more water resistant are disclosed in U.S. Pat. No. 4,396,453.
Styrene-butadiene co-polymer latexes, which are available commercially, have also been utilized in adhesives, coatings and other uses. For instance, U.S. Pat. No. 4,396,453 also discloses a water-resistant starch-based adhesive comprising corn or wheat starch, a carboxylated styrene-butadiene latex, a thickener and a cross-linking agent. U.S. Pat. Nos. 4,584,501, 4,568,714, 4,812,496 and JP 57143371 disclose variations on this starch-based adhesive formulation. That is, incorporation of a protein, such as casein, to provide improved bonding to coated surfaces or to surfaces of low porosity; replacement of the carboxylated styrene-butadiene latex with an ammonia-based latex, such as ammonium based styrene-butadiene latexes and carboxylated styrene-butadiene latexes, to improve the wet-pin strength; and adjustment of the pH to within 4 to 7 with addition of liquid urea resin to provide improved water resistance are disclosed.
Other prior art paperboard adhesive formulations that contain a styrene-butadiene latex can be found in U.S. Pat. No. 3,985,937, which discloses a water-resistant paperboard laminating adhesive comprised of either a polyvinyl acetate or a styrene-butadiene latex emulsion to which clay and other additives, such as paraffin wax and a polyglycol humectant and/or wetting agent, are added, and JP 52133338, which achieves water resistance by combining a wax emulsion with either a polyvinyl acetate emulsion, an alkali starch solution or a styrene-butadiene latex. Japanese patent 139,158/82 discloses that a starch-based adhesive homogenized with polystyrene latex (glass transition temperature 80.degree. C.) has a greater bonding strength compared to the starch-based adhesive homogenized with styrene-butadiene rubber latex (glass transition temperature 20.degree. C.).
Thus, styrene-butadiene latex has been added to other adhesive formulations, such as starch-based formulations, to impart strength and water resistance to the formulation. However, prior to this invention, the use of an adhesive substantially comprised of styrene-butadiene latex in an aqueous carrier; such as an adhesive in which the viscosity is less than 1000 centipoise and in which at least 80% by weight of the solids content comprises styrene-butadiene latex, was not considered suitable as a paperboard adhesive.
Despite the availability of many paperboard adhesives formulated for strength, a common failure point in paperboard products is a glue joint, such as handle glue joints in paperboard sleeve-and basket-style cartons and the like. Thus, the limiting factor in the construction of many paperboard products is the end bond strength of an adhesive. There exists a need, therefore, for an improved paperboard product, and a method of making the same, that possesses strengthened glue joints. There exists a further need for a :method of forming paperboard products with strengthened glue joints using high-speed gluing facilities, such as straight-line gluers.
Surprisingly, it has now been found that application of an adhesive consisting primarily of a styrene-butadiene latex (or a modified styrene-butadiene latex) in an aqueous carrier, particularly in combination with the separate application of a green-strength adhesive in a manner such that the green strength adhesive does not substantially overlap the styrene-butadiene latex adhesive in the resulting paperboard product, provides a paperboard glue joint that not only is water resistant but possesses considerably enhanced bonding strength and can be applied using high-speed gluing machinery. Because particular paperboard joints, such as those found in the handles of paperboard cartons, are likely to be the points of failure in a paperboard product, this new discovery not only allows for strengthened paperboard products but also allows for paperboard products constructed from thinner paperboard or recycled paperboard. The enhanced strengthening of the paperboard bond is thought to derive from the penetration of the styrene-butadiene latex adhesive into the paperboard, perhaps forming a composite-like structure with the paperboard.
Furthermore, although separate application of more than one glue to a paperboard surface to form a strengthened adhesive bond is known in the art, the synergistic results of separately applying a styrene-butadiene latex adhesive and a green-strength adhesive as described in this invention are unexpected.